The criticism is due to the 5mm thinness hype, when they're actually not far off the size of the old ones, it's just the edges are tapered out. It's like if I pinched a bit of flab at the side of my waste and said I'm 5mm thin.
Also, from close up the "thickness is almost impossible to gauge" due to the trick of perspective. However, they do look fatter from a distance, notably when Phil Schiller coyly turned the new imac to it's side he seemed aware that the distant cameras would pick that up - he had been avoiding turning it more than a few degrees round while he praised the thinness.
The reality is, it's not that much thinner. But that's a good thing - for one, we still want some decent hardware in there (which it has), but what is really excellent about the design is that the removal of the optical drive has allowed for a single fan design which directs air to the back of the bulge.
It does look very very nice. I can't resist upgrading to this model myself. But I do understand why people are mocking the form. Apple massively over-hyped the thinness, so people are just having a bit of fun teasing.